EP-A-1303719 (filed by the present applicant) describes an electromagnetically operated device of the form shown in present FIG. 1.
A linear armature 10 is sandwiched between two internal pole pieces 12, 14 which are themselves sandwiched between two permanent magnets 16, 18. Two external members 20, 22 extend beyond the end of the sandwich of magnets and at their remote ends provide two inwardly facing poles 24, 26.
The two internal pole pieces 12, 14 extend beyond the permanent magnets 16, 18 where they are surrounded by a coil former 30 on which a coil 32 is wound. Two cylindrical inserts 34, 36 are fitted in the poles 24, 26 and the opposed inner ends of the inserts define orifices 38, 40.
The armature can be deflected and a magnetic field induced by a current flowing in the coil 32. Depending on the direction of current flow in the coil, the armature will be attracted to the pole 26 and insert 36, as shown in solid outline in FIG. 1, or towards the other pole 24 and insert 34, as shown in dashed outline.
In the known configuration shown in FIG. 1, there are regions 40, 42 of significant magnetic flux leakage beyond the external dimensions of the device. These regions also extend horizontally around the device above and below the plane of the drawing.
This is likely to be problematic in applications where the device is surrounded by fluid, as there will be a risk of build up of magnetic debris in these regions potentially leading to flow impairment or even blockages. Furthermore, the flux leakage may interfere with the operation of other adjacent devices. Similarly, the device may be susceptible to the influence of external magnetic fields.